The present invention is related to reduced toxicity fuels, and more particularly, to reduced toxicity fuels containing hydrocarbons having both strained rings and internal, conjugated triple bonds. The disclosed fuels are hypergolic with nitrogen tetroxide and in some cases with inhibited red fuming nitric acid as well.
Hypergolic propellants are combinations of fuels and oxidizers that ignite spontaneously upon contact with one another and require no ignition source. The extremely rapid, reliable start and re-start capability of hypergolic propellants make them ideal for spacecraft maneuvering systems. In addition, since hypergolic propellants remain liquid at ordinary temperatures, they do not pose the storage problems of cryogenic propellants.
The hypergolic fuels currently in widespread use are highly toxic and must be handled with extreme care. Examples of these fuels include hydrazine, monomethylhydrazine (MMH) and unsymmetrical dimethylhydrazine (UDMH). The oxidizers typically used with these fuels to provide a hypergolic bipropellant system include nitrogen tetroxide (N2O4) and nitric acid (HNO3). UDMH is used in many Russian, European, and Chinese rockets, while MMH is used in the orbital maneuvering system (OMS) and reaction control system (RCS) of the Space Shuttle orbiter. The Titan family of launch vehicles and the second stage of the Delta rocket use a fuel called Aerozine 50, a mixture of 50% UDMH and 50% hydrazine.
Drawing on the German Wasserfall rocket of World War II, nitric acid (HNO3) became the early storable oxidizer of choice for missiles and upper stages during the 1950's. To overcome various problems with its use, it was necessary to combine nitric acid with nitrogen tetroxide (N2O4) and various passivating agents that protect metal tanks and engine components from corrosion (typically small amounts of hydrogen fluoride or free halogens). The presence of these passivating corrosion inhibitors was denoted by the designation “inhibited red fuming nitric acid,” which is commonly referred to by its acronym, “IRFNA.” Unreliable ignition and unstable combustion led to the abandonment of many early propellant combinations, but the combination of IRFNA and UDMH did finally prove satisfactory.
The compositions of propellant-grade nitric acids are described by Military Specification MIL-N-7254. They are all fuming liquids with a density of about 1.5 grams per cubic centimeter and vary in appearance from colorless to dark brown, depending on the amount of dissolved N2O4. The vapors from these acids have a characteristic pungent odor. They are highly corrosive, oxidizing agents that attack most metals and react with some organic materials with sufficient vigor to cause fire. These acids are miscible with water in all proportions (with accompanying evolution of heat) and cannot be made to explode. Approximately 90 percent of the nitric acid is made by the catalytic oxidation of ammonia with air or oxygen to yield nitric oxide (NO). The latter is oxidized to N2O4 which, when treated with water, yields nitric acid (HNO3) and may be concentrated by distillation with sulfuric acid. Red fuming nitric acids can be also prepared by passing gaseous N2O4 into nitric acid.
By the late 1950's it was apparent that nitrogen tetroxide by itself was a better oxidizer, and nitric acid was largely supplanted by pure N2O4 in storable liquid fuel rocket engines developed after 1960. Nitrogen tetroxide consists principally of N2O4 in equilibrium with a small amount of nitrogen dioxide (NO2). The purified grade contains less than 0.1 percent water and has a density of 1.45 grams per cubic centimeter. Nitrogen tetroxide boils at 21° C. and has a characteristic reddish-brown color in both the liquid and gaseous phases; the solid phase (melting point −11° C.) is colorless. N2O4 has an irritating, acrid, acid-like odor and is a very reactive oxidizing agent. Although it is non-flammable with air, it will inflame many combustible materials. It is not sensitive to mechanical shock, heat, or detonation. Commercially, nitrogen dioxide is produced by the catalytic oxidation of ammonia. Steam is used as a diluent to reduce the combustion temperature. Most of the water is condensed out and the gases are further cooled; the nitric oxide is oxidized to nitrogen dioxide, and the remainder of the water is removed as nitric acid. The gas is essentially pure nitrogen tetroxide, which is condensed in a brine-cooled liquefier.
Monomethylhydrazine (CH3NHNH2) is a storable liquid fuel that found favor in the United States for use in orbital spacecraft engines. Its advantages in comparison to UDMH are higher density and slightly higher performance. Monomethylhydrazine (MMH) is a clear, water-white hygroscopic liquid with a density of 0.88 grams per cubic centimeter that freezes at −52° C. and boils at 87° C. It tends to turn yellow upon exposure to air. MMH is a toxic, volatile liquid that reacts with carbon dioxide and oxygen. It has the typical sharp ammoniacal or fishy odor of amines, and is highly toxic.
Unsymmetrical dimethylhydrazine (1,1-dimethylhydrazine; (CH3)2NNH2) became the storable liquid rocket fuel of choice by the mid-1950's. Generally known by its acronym “UDMH,” it is used in virtually all storable liquid rocket engines except for some orbital maneuvering engines in the United States, where MMH has been preferred due to its slightly higher density and performance. Unsymmetrical dimethylhydrazine of 98 to 99% purity is described by Military Specification MIL-D-25604. It is a clear, hygroscopic liquid with a density of 0.79 grams per cubic centimeter that freezes at −57° C. and boils at 63° C. Like monomethylhydrazine, it exhibits the sharp ammoniacal or fishy odor characteristic of organic amines.
Both unsymmetrical dimethylhydrazine and monomethylhydrazine are confirmed animal carcinogens that have been characterized as tumorigenic, mutagenic, reproductive effectors. The Occupational Safety and Health Administration (OSHA) limits permissible exposure to only 0.5 parts per million (1 milligram per cubic meter) on an eight hour time-weighted average.
This extreme toxicity and the associated difficulties and expense of transporting and handling these compounds have produced widespread interest in finding replacement fuels. This search is the subject of ongoing formal programs conducted both by NASA and various branches of the US military.
U.S. Pat. No. 2,693,077 to Malina et al., discloses fuels spontaneously combustible with oxidizing agents like red fuming nitric acid. Among the fuels described as being useful are liquid hydrocarbons containing a large fraction of unsaturated carbon bonds. The only specific examples cited are divinyl acetylene, dipropargyl, and propargyl alcohol. However, divinyl acetylene is an extremely hazardous substance due to its thermal instability, shock sensitivity, and tendency to spontaneously polymerize, forming a resin that explodes when handled. Dipropargyl(dipropinyl or 1,5-hexadiyne), isomeric with benzene, is a flammable, pungent-smelling mobile liquid that is thermally unstable. Propargyl alcohol (2-propyn-1-ol) is flammable, toxic, a suspected carcinogen, and also tends to polymerize.
U.S. Pat. No. 6,272,846 to Schneider discloses that: “Reduced-toxicity fuels have not been used in the past, due to the fact that candidate fuels are not hypergolic. In other words, liquid reduced toxicity fuels will not spontaneously react with an oxidizer to begin the combustion process as in prior art fuels such as hydrazine.” Likewise, U.S. Pat. No. 6,311,477 to Schneider again notes: “Unfortunately . . . reduced toxicity propellants suitable for use with satellite propulsion are not hypergolic.”